Numbers enclosed in brackets throughout the specification refer to correspondingly numbered items in the List of References at the end of the specification. The disclosures of these items are incorporated herein by reference.
Multi-band, multi-standard radio receivers such as next generation 7-band cellular phones and the joint task force radio system (JTRS) require a large array of channel-select filters connected in parallel. The input capacitance of the filter array will ‘load’ individual filters, deteriorating their stop-band rejection. Therefore, such frequency agile radios need multi-octave tunable band-select radio frequency (RF) filters and bandwidth tunable channel-select intermediate frequency (IF) filters with good shape factor and excellent stop-band rejection. An IF filter with dynamically tunable bandwidth will enable handling of multiple waveforms, eliminate out-of-channel interferers, and substantially decrease the number of filters in next-generation receivers.
Dielectrically-transduced thickness shear-mode resonators with analog voltage tunable center frequency and bandwidth are suitable candidates for channel-select IF filters [1]. The resonators require a back-side etch of the SOI substrate to pattern orthogonal frequency tuning electrodes and eliminate parasitic pad capacitance and resistive ground loops. However, such back-side processing is not compatible with the high-vacuum, ultra-clean epi-silicon encapsulation technology [2] necessary for field deployment of these filters. Contour-mode MEMS resonators with Q>5,000, low RX, compatibility with epi-silicon encapsulation and CAD-defined resonance frequencies from 10 MHz-1 GHz are excellent candidates for designing channel-select filter arrays [3].
Unlike thickness shear mode resonators, however, the frequency expressions for contour modes and flexural vibration modes do not directly couple. It is therefore difficult to perform orthogonal frequency tuning of contour-mode resonators. However extensional mode resonators cannot be tuned at device-level without excessive heating [4] or using liquid dielectric [5]. Therefore, it would be desirable to have a practical, easily manufacturable, and digitally programmable RF MEMS filter that overcomes such existing limitations.